Method and arrangement for treating chemical pulp

ABSTRACT

A method to pulp cellulosic fibrous material including: alkaline cooking cellulosic fibrous material in a continuous digester vessel to produce a pulp having a kappa number in a range of 50 to 120; washing the pulp discharged from the digester vessel; treating the washed pulp in the presence of oxygen and alkali to remove lignin; washing the oxygen-treated pulp such that the kappa number is recued by at least at least 30; separating lignin from wash filtrate generated by the washing of the oxygen-treated pulp, and using the separated wash filtrate as a washing liquid in the washing of the pulp from the digester vessel.

The present invention relates to a method and an arrangement fortreating chemical pulp when the pulp has been cooked to a high kappa.

The kappa number indicates the residual lignin content and it is used atall mills as a target value for cooking. A typical kappa number for pulpto be bleached is at present 14-20 for hard wood and 25-35 for softwood. If the pulp is not bleached, the kappa number after cooking isclearly higher, typically 40-100. Soft wood pulp to be bleached istypically treated in an oxygen stage for decreasing the kappa number to10-20.

In connection with the invention, a high kappa pulp refers to kappanumbers exceeding 50. High kappa pulps (high yield pulps) are usuallyused in products, from which high strength properties and rigidity arerequired, but not brightness or printing properties. For this reason,soft wood is a typical raw material for high yield pulps due to its longfibers. Soft wood has a high lignin content, for which it requires arelatively long chemical treatment. A combination of strength propertiesand rigidity is typical for soft wood pulp. For high yield pulps, thelignin content remains so high that in prior art technique the pulps cannot be defibrated without mechanical treatment, but in this case thefinal product does not require printing properties or brightness.

High yield pulps are typically produced in a sulfate process. The yieldrange of high yield pulps varies in the range of 50-70%. It is commonknowledge that this kind of pulp is not produced using fiber lineequipment designed for pulp with a normal kappa number, but additionaltechnique is required. The most important difference between the fiberlines is that high kappa pulp after cooking is in such a strong chipform that the defibering thereof requires special defibering equipment,such as a blow line refiner (In-Line refiner), or alternatively hotrefining. The arrangements are of similar type. In the first-mentioneddefibering arrangement, cooked pulp in the form of chips is dischargedfrom the digester into a refiner located in the blow line upstream ofwashing stages. The aim of the defibration of cooked pulp is to separatethe fibers from each other, since after cooking the fibers are still inthe chip matrix. This defibration allows producing pulp to a high kappaand to high yield values. Even a slight defibration is mechanicaltreatment of the pulp, the effects of which in alkaline conditionsdecrease the quality of the pulp.

High kappa pulp is produced to so-called brown stock. Thus, there is noaim to bleach this kind of pulp. The production line for this pulpcomprises washing and screening of the pulp. If any special propertyrequirements are set for the pulp quality, the line may comprise anoxygen stage for adjusting the properties of the pulp to a desiredlevel.

Dissolving of carbohydrates (cellulose and hemicelluloses) duringcooking causes a remarkable increase in the raw material costs, sincecarbohydrate losses in both the beginning and at the end of cooking arebig. One way of decreasing the decomposing of carbohydrates is to endthe cook at a higher kappa number. Typically, the yield is increased, ifthe cook is ended at a high kappa number and the pulp is bleached withchlorine dioxide without an oxygen stage. Still, the costs of bleachingand increasing effluent discharges exceed the allowed limits. Keepingthe effluent discharges at the BAT (Best available techniques) milllevel requires the use of an oxygen stage, whilst so that thecarbohydrate yield is the highest possible. Vigorous development ofoxygen delignification has provided an efficient method of decreasingthe kappa of pulp to be bleached without substantial losses in yield.The present trend has been to stop reaching cooking kappa numbers ofremarkably low level.

Important in sulfate cooking is selectivity, i.e. when lignin isdissolved, a least possible amount of carbohydrates (cellulose andhemicelluloses) is split and dissolved. A problem is caused by chemicalreactions, due to which a remarkable amount of hemicellulose isdissolved at the finalizing stage of the cook. Thus, at the final stagedissolving of carbohydrates also causes a decrease in the strength ofchemical pulp. In the case of softwood pulp, the strength of chemicalpulp is one of the most essential quality indicators. In addition topulp properties, also process yield is an important cost factor. The aimin the past years has been to find such pulping methods, by means ofwhich the pulp yield from wood would be the highest possible. It hasmeant either increasing the selectivity of the cook or cooking only tosuch a kappa number that the use of more selective delignificationmethods provides remarkable yield benefit and thus economical importanceThe differences between sulfate cooking methods in a final kappa range(for softwood 25-40) are so small that no substantial differences interms of selectivity are obtained by means of them. In practice, afeasible method for increasing the yield is to finish the cook at aclearly higher kappa number level, where the carbohydrate yield isrelatively higher than in cooks of pulp to be bleached and to continuethe delignification with oxygen, which is known to be a more selectiveway of delignifying pulp compared to cooking.

Due to alkaline conditions, oxygen delignification is a directcontinuation to lignin-removal taking place during cooking. In theindustrial cooking processes the chemical pulp is not cooked tosignificantly below kappa number 25, since the yield loss would beessential. On the other hand, there is a desire to optimize the cook toa highest possible kappa number in order to maintain the reject contentunder control and the pulp bleachable. Between the cook and the finalbleaching, oxygen delignification is a more selective and gentlerprocess. It disintegrates and oxidizes lignin to a form that isdissoluble in alkali, destroys colored compounds present in lignin andremoves impurities (resin) from the pulp.

In view of quality, defibrating the pulp after discharge from thedigester in a blow line defibrator is never the so-called optimalsolution, but defibrating the pulp chemically without remarkablemechanical loading preserves the fiber properties best.

The latest cooking studies have revealed that cooking can produce highkappa pulp that is defiberazed to such an extent that this chip-formpulp can be pumped and treated in a fiber line even at a kappa number of70-90 (Jiajun Wang, Master's Thesis, Aalto Yliopisto, School of Scienceand Technology 2010; WO Patent Application PCT/US10/57417). Also in thisstage, the cooked pulp contains abundantly of analyzed reject, but thecharacter of the reject is such that is does not limit the pumping ofthe pulp to washing and subsequent process stages.

FI patent application 20115277 presents a method of treating pulp thatis produced by chemically cooking soft wood chips to a kappa number of50-100, preferably 60-90, in which method

-   -   the cooked pulp is led to washing without treatment in a        mechanical defibrator between the digester and the washing, the        pulp is washed,    -   the washed pulp is led into a first oxygen delignification stage        where the pulp is treated in the presence of oxygen and alkali,    -   the oxygen delignified pulp is washed and after the washing it        is treated in a subsequent stage with an electrophilic chemical,        after which    -   the pulp is washed and treated in a second oxygen        delignification stage with oxygen and alkali, and the thus        treated pulp is washed and led to further treatment into a        bleaching process.

Oxygen stage in connection of the present patent application refers tosuch an alkaline stage that takes place pressurized within a pressurerange of 1-20, preferably 8-12 bar (abs.) at the mixing point, at a pHrange of 8.5-14 and wherein at least during part of the reaction timeoxygen is present around the fibers. The oxygen stage can comprise one,two or even several steps, whereby each reaction step comprises chemicalmixing and a reaction vessel or a reaction retention accomplished bymeans of a tube. In practice, treatment step here refers to addition andmixing of a chemical used in the oxygen stage and the subsequentretention in a portion of the tube or the reactor. Reaction retentionsare, depending on the practicing method, from 0.1 minutes up to 120minutes, so that the reaction retention depends on the desired type ofreactor. In this connection, an oxygen stage is also known from awashing stage both upstream and downstream of the oxygen stage.

Most usually, oxygen and alkaline and possibly some inhibitor preventingfibers from getting damaged by metals are dosed into the oxygen stage,or metals entrained in the fibers are otherwise removed or treated tobecome inactive. Alkali is generally dosed in the amount of 1-60 kg/admtand oxygen 1-50 kg/admt. When the alkali dose is 0.8-1.4×kappa unit andthe oxygen dose 0.7-1.3×kappa unit, it can be noted that the alkali doseof the oxygen stage may be up to 90 kg/adt and the oxygen dose 70kg/adt, when the kappa reduction in the oxygen stage is 60 units. Thealkali that is used is most often sodium hydroxide or oxidized whiteliquor, but in principle all alkaline compounds containing OH-ion arealkalies that could be used in the oxygen stage under some conditions.The make-up sodium hydroxide of the chemical cycle is oftenadvantageously dosed in the oxygen stage. The oxygen is dosed in gaseousform, where the content of oxygen is most usually 75-100% of specificgravity. The temperature of the oxygen stage is 70-120° C. and mostusually 80-105° C. The temperature can be raised using some suitablesteam having a pressure of 0.5-20 bar, and hot water either via washingor diluting. Steam can be used for heating either directly, mixed in thepulp, or indirectly.

In the oxygen delignification of high kappa pulp, especially in thefirst treatment, a large amount of lignin is released (the kappa numberdecreases by e.g. 30-70 units), which is to be taken into account in theprocess. When the amount of removed lignin is e.g. 40 kappa units, theamount of organic material being released from the pulp becomes high.With said kappa reduction, the proportion of e.g. lignin only isapproximately 7% of the dry substance of the pulp, and theretocarbohydrate losses. Said amount of lignin raises the COD-amount of thefiltrate of the oxygen stage by 60-120 kg/adt and the amount of drysubstance in the same proportion. The oxygen stage is typicallyconnected in accordance with the counter-current washing principle sothat the function of the washing of the pulp arranged between thecooking and the oxygen stage is to displace the liquor entrained in thepulp coming from the washing, which liquor can be separated as afiltrate of the washing stage of the cooked pulp, with a filtrateobtained from the oxygen stage washing. Thus, the COD of the washingliquid of the cooked pulp will be in the range of 30-60 g/l and that ofthe pulp suspension in the range of 200-500 kg/adt at a consistency of12%. Generally, filtrates from pulp washing are led counter-currently inthe fiber line, but treating an oxygen stage filtrate having a highCOD-value this way is not necessarily advantageous. Industrially knownCOD-levels are clearly lower than the above-mentioned in the studiesthat have been conducted for determining the behavior of filtrategenerated from the oxygen stage, when it is led counter-currently to thesame stage. The above presented levels are already so high that theyhamper the operation of the oxygen stage and cause consumption ofchemical together with the circulating lignin. When outlining the oxygenstage for high kappa pulp, the amount of reaction products is to betaken into account.

Because the reaction products comprise, due to the selective characterof the oxygen stage, lignin and a minor amount of carbohydrates, it canbe stated that in the washing filtrate of the oxygen stage theproportion of lignin and carbohydrates is higher than in black liquor.Thus, the use thereof in washing of cooked pulp according to thecounter-current principle is not advantageous without disturbing theoxygen stage process. Accordingly, a clear need exists to outlinecouplings of washing and oxygen stage for pulp cooked to a high kappa ina novel way, which is in many ways more practicable.

The present invention relates to a method of treating chemical pulp, inwhich method pulp is produced by cooking wood chips chemically to akappa number of 50-120, preferably 60-100, the cooked pulp is washed,the washed pulp is led into an oxygen stage where the pulp is treated inthe presence of oxygen and alkali for removing lignin, and theoxygen-treated pulp is washed, whereby lignin-containing washingfiltrate is formed. The method is characterized in that

-   -   in the oxygen stage the kappa number of the pulp decreases by at        least 30 units, and    -   the filtrate of the washing downstream of the oxygen stage is        treated for separating lignin, and the treated filtrate is led        as washing liquid into the washing apparatus for the cooked        pulp. The E₁₀-value of the washing apparatus for cooked pulp is        typically at least 8, preferably at least 10.

The so-called E10-value defines the washing efficiency so that washersof different types can be compared to each other. The E10-value is anumerical value defined for a washer or a combination of severalwashers, which reveals how many ideal mixings the washer or washercombination reaches. An ideal mixing, in turn, is understood as asituation when the washing liquid is mixed into the pulp being washed soefficiently that the concentrations of both the liquid remaining insidethe pulp and the liquid being withdrawn therefrom are equal. Number 10in the E₁₀ value reveals the calculated consistency percent, at whichthe pulp exits the washer. Dilution factor means the difference betweenused washing liquid and liquid exiting the washing apparatus with thepulp per a ton of pulp.

The invention also relates to an arrangement for treating pulp producedfrom wood chips in a cooking apparatus wherein the wood chips are cookedchemically to a kappa number of 50-120, preferably 60-100, saidarrangement comprising at least

-   -   a cooking apparatus for cellulose fibrous material, washing        devices for cooked pulp,    -   apparatuses downstream of washing of the cooked pulp for        delignifying the pulp in an oxygen stage,    -   apparatuses for washing the pulp after the oxygen stage, and    -   filtrate lines for leading washing filtrates counter-currently        to preceding washers as washing liquid. It is essential that a        filtrate line downstream of the oxygen stage is provided with        devices for separating lignin from the filtrate and that the        filtrate line is connected to the washing devices for cooked        pulp for using the filtrate as washing liquid after lignin        separation.

The E10-value of the washing devices for cooked pulp is preferably atleast 8, preferably at least 10. The washing devices for cooked pulptypically comprise more than one stage. The washing devices for cookedpulp typically comprise a so-called digester washing and at least onewashing device between the digester and the oxygen treatmentapparatuses.

The washing devices for cooked pulp can comprise as one stage a digesterwash conducted in the lower part of the digester, such as a hi-heat typeof digester displacement or final displacement taking place in batchcooking. The washing devices for cooked pulp comprise downstream thereofone or several washers, such as preferably a single drum washer, forinstance a Drum Displacer® (DD) washer (Andritz Oy) or a diffuserwasher. If the cooked pulp does not contain uncooked fraction in harmfulamounts, such as knot, also a press can be used, under the preconditionthat the E₁₀-level of the overall washing devices for cooked pulp isover 8. The dilution factor of the washing devices is to be 0-5,preferably 1-3. Adequate efficiency of washing is important in order toensure adequate removal of lignin released from the pulp during cooking.More than 95%, preferably more than 97% of lignin is removed in thewashing. Simultaneously, also sulfur can be removed from the pulp.

The present invention complement the performance of bleaching pulpdelignified at a high kappa so that it becomes industrially possible.Additionally, it allows pulp production to a significantly higher yieldthan in a conventional fiber line. Further, sulfur-poor lignin can berecovered from the process even though the pulp is produced by thesulphate method using a conventional fiber line and recovery concept.The invention is more preferably applied to soft wood in view of ligninamount and cooking/delignifying properties, but it can also be appliedto hard wood, especially hard wood species having a high lignin-content.

In the presented method, the pulp is produced by an alkaline cookingmethod, such as a sulphate cook, a soda cook or a soda-anthraquinonecook. The cooking method may also be modifications of these. A preferredexample is a sulphate cook having a low sulfidity, i.e. 5-25%. Sulfidityvalues lower than that relate to soda cooking or modifications thereof.The cook may be a continuous cook or a batch cook.

In the cook, the pulp is cooked to a kappa number of 50-120, preferably60-100. This kind of pulp has approximately 7-14 weight percent lignin,which is an essentially significant amount. This lignin is removed inthe oxygen stage so that the kappa number of the pulp will be typicallyapproximately 20-40 and approximately 50, if the kappa of cooked pulp is100-120. Thereby, a significant amount of lignin in view of botheconomical and process technology aspects is transferred into the oxygenstage filtrate. According to an embodiment of the invention, the oxygenstage is not carried out using oxidized white liquor, but preferablyusing sodium hydroxide, whereby the lignin is released in the solution,which is sulfur-poor compared to an oxygen stage carried out usingoxidized white liquor. Sulfur from the magnesium sulphate used as aninhibitor and from cooking chemicals enters the oxygen stage. In thiscase, it is more advantageous to use in the washer upstream of theoxygen stage a liquid devoid of sulfur compounds generated from oxidizedwhite liquor. Lignin that has less sulfur than normal is advantageous inmany further use applications for lignin. The method according to theinvention does not require or perform any separate chelate treatmentprior to the oxygen stage. Thus it is possible to decrease the amount ofeffluent fractions and process costs.

After the oxygen stage, the pulp is washed normally so that reactionproducts are separated from the pulp in a liquid phase. It is possibleto produce lignin in the amount of 40-120 kg, typically 40-80 kg, perton of pulp. In the invention, the alkali used in the oxygen stage ispreferably sulfur-free or sulfur-poor, whereby, when efficiently washingthe pulp both downstream and upstream of the oxygen stage using liquidessentially free of sulfur compounds, it is possible to receive alignin-containing liquid having sulfur-poor lignin in the amount of40-100 kg per ton of pulp. The monetary value of this lignin is sosignificant that it is worth recovering, especially since leading lignincompounds to washing upstream of the oxygen stage is such a majordisturbance factor for the process that it does not have processtechnical or economical grounds. Low-sulfur lignin refers to ligninhaving a sulfur-content less than 1% of the dry matter in the lignin.According to a study, the sulfur-content of precipitated from blackliquor was approximately 1.4% (sulfur) of the dry matter and thesulfur-content of lignin precipitated from the oxygen stage washingfiltrate was approximately 0.66% of dry matter.

In the solution presented herein, filtrate of the oxygen stage isseparated from the pulp and lignin is recovered therefrom e.g. to beused as raw material for chemical industry. Correspondingly the liquid,wherefrom the lignin has been separated, can be re-used in the oxygenstage such that it is led to washing upstream of the oxygen stage,without harmful effect of lignin. Thus, removal oxygen reaction productsof the oxygen stage from a counter-currently connected washing cycle anda separation technique that is in any case to be connected to the oxygenstage of high-kappa pulp can be regarded as investments having a cleareconomical target, and not only as additional cost for ensuring thefunctioning of the process.

In order to optimize the sulfur-content of lignin, the alkali of theoxygen stage is to be either pure sodium hydroxide (NaOH) or some otherhydroxide that enables maintaining an adequately high pH-level in theoxygen stage. Pure here means that the sodium hydroxide solution doesnot contain any sulfur, or contains very small amounts of sulfur.Preferably the oxygen stage chemical is not a chemical that containssulfur compounds. One chemical typically used in the oxygen stage ismagnesium sulfate as an inhibitor. The use thereof can be optimizedaccording to the pulp quality requirements and according to how low thesulfur-content is to be. Though oxidized white liquor is preferably notused, it is still possible in connection with chemical regeneration at achemical pulp mill to separate the alkali so that one fraction containsthe sulfur compounds and another the pure NaOH. In this case, thechemical proportion is formed so that the sulfidity in the cook is over40%. The sulfidity returns to “normal” in the stream going to blackliquor evaporation plant, when in accordance with the counter-currentprinciple the cooked pulp is washed using low-sulfur washing liquid fromlignin separation. In another embodiment the sulfidity of the cook isapproximately 25-40%, but that of liquor in recovery it is clearlylower, e.g. 25-30%.

Prior art has presented several processes for fractionating green liquorto sodium hydroxide and sulfur-containing fractions in connection withchemical regeneration. This kind of processes are known e.g. fromFI104334, FI98226 and U.S. Pat. No. 5,607,549. The sodium hydroxide isused in the oxygen stage and the sulfur-containing fraction in the cook.Then there is no need to use purchased chemical as the alkali in theoxygen stage, but the required chemical can be produced at the mill inconnection with chemical circulation.

According to an embodiment, also oxidized white liquor can be used inthe oxygen stage. Then, in the production of white liquor, the sulfideis oxidized with oxygen via thiosulfate to sulfates. This is not a veryadvantageous embodiment, if minimizing the sulfur-content of the ligninis desired.

The lignin separation method as such is not essential in view of theinvention, but before all the fact that lignin separation becomesprofitable and that according to a preferred embodiment the lignin canin a kraft-process be separated in low-sulfur form. Then the process isto comprise the following properties:

Kraft-cook to kappa 50-120.

Efficient washing of pulp after cooking and the oxygen stage. TheE10-value of the washing devices for cooked pulp at least 8, preferablyover 10. The E10-value of the oxygen stage washer over 4, preferablyover 7. The required washing efficiency is also dependent on howaccurately the cook-generated lignin and the oxygen stage lignin areseparated in the washes. The more efficient the washing of the cookedpulp, the better the cook-generated lingnin is separated and the moreoxygen stage-generated lignin can be separated in the oxygen stagewashing in low-sulfur form.

-   -   The washing liquid introduced to the last stage of the wash        upstream of the oxygen stage is liquid devoid of essential        amounts of sulfur.    -   The alkali during the oxygen stage is preferably sulfur-free,        when very low-sulfur lignin is desired.    -   In the oxygen stage the kappa is decreased by at least 30 units,        preferably by more than 35 units, for releasing from the pulp an        economically significant amount of lignin.

A significant portion of the lignin of the pulp is dissolved in theoxygen stage into the liquid traveling with the fibers and it is thuspossible to separate from the filtrate of the washers or the presses.When the lignin has been separated, the thus purified liquid can be usedin the washing of cooked pulp. The washed pulp is taken either to boardproduction, as such or it can be further bleached in a method presentedalso in this patent application.

Since a modern chemical pulp mill can already be almost totally closedas to sulfur and the mill is in practice free of sulfur losses, it isclear that the washing of cooked pulp can removed sulfur highlyefficiently and thus a conventional washing technique with an E₁₀-valueover 8, is suitable for removing sulfur from pulp.

Essential in connection with the present invention is that pulp that hasbeen cooked to a significantly high lignin-content can be processed forthe market to pulp that can also be washed to a desired cleanlinesslevel. This opens a totally new possibility for building a biorefineryand applying various treatment methods, as a result of which lignin canbe separated from fiber slurry. Naturally, possible embodiments areoften limited to very affordable chemicals, because the process is to bemade profitable, but still several various methods exist that can beapplied for lignin separation.

Separated lignin can also be used as fuel in the boilers and kilns of achemical pulp mill.

During the last years, the wood processing industry and also thechemical industry have awakened to search substitute raw materials forproducing polymers and other carbon-based materials. At present, themost important raw material of polymer industry is crude oil, the priceand availability of which in the long run will make wood-based rawmaterials for polymers competitive. Thus, also lignin based on achemical pulping process has market where it can replace crude oil-basedproducts. As the most important chemical pulp production processes arebased on the use sulfur-based cooking chemicals, the usability of ligninseparated from those is in the chemical industry low or restricted, ifsulfur-removal from lignin is not taken care of. And if the cookingprocess is accomplished using sulfur-free chemicals, the quality oflignin is improved, but the pulp production assumes many such featuresthat the processes have not yet become widely competitive. An advantageof the present invention is that it produces lignin suitable of furtherprocessing.

The oxygen stage is the simplest and best known method for delignifyinghigh kappa pulp. Then the pulp is treated with a method in which fiberproperties are the priority and the objects of use of the fiber rawmaterial are known paper and fiber products. It is also possible to useother lignin dissolving and separation method, replacing the oxygenstage, which are used to produce e.g. special lignins or to separatee.g. hemicelluloses. This kind of methods could comprise an acid pulpingmethod, such as a defibration suitable for grass pulp presented byChempolis, a formic acid pulping known as the CIMV-process, or acombination of acid hydrolysis and an oxygen stage. In this relation,there are several alternatives, since high kappa pulp (kappa e.g. 100),from where cooking chemicals, first of all sulfur, have been removed bywashing, is raw material for numerous different products. Thus, ligninis not the only possible product, but in addition to it, the productsmay comprise e.g. hemicelluloses.

A solution that is separated from the pulp in the oxygen stage can betreated in many ways for separating lignin. The most conventionalchemical separation method is precipitation, where an alkaline solutionis neutralized or acidified, whereby alkali is precipitated from thesolution and it can be clarified out of the liquid. This requires anabundant amount of acid, but on the other hand it is an efficient andsimple way of recovering lignin. About 15% of the lignin is precipitatedat a pH of 10 and 50% at a pH of 8. The pH of the oxygen stage is 10-13,typically 10.5-12. In lignin separation there is no need to aim atcomplete separation. A satisfactory result is obtained in the washing ofcooked pulp even if only approximately 50% of the lignin is separatedfrom the oxygen stage washing filtrate prior to using it as washingliquid. On the other hand, circulating lignin via the oxygen stagedeteriorates its quality so that it is advantageous to aim at a higherseparation degree. Greater amount of native lignin in the recoveredlignin is advantageous in view of further processing.

Other lignin separation ways comprise various filtration methods, inwhich a filter can remove large chained molecules, but lets ions andwater go through. This kind of methods comprise various ultra- ornanofiltrations, osmosis and reverse osmosis, dynamic crossflow-filtration, and other methods by means of which molecules can beseparated from a solution. When the lignin has been separated from thesolution, the cleaned solution can be utilized in the washing of cookedpulp, because the organic loading has been removed.

When looking at a complex, where this kind of solution is suitable, suchadvantages show up, which as individual steps introduce remarkableimprovements in the economical potential of the mill:

-   -   The pulp yield of the fiber line is clearly better.    -   A solution is obtained by means of which the oxygen stage        functions well when high kappa pulp is treated.    -   A process is formed where separation of low-sulfur lignin is        possible.    -   The whole chemical cycle of the process, such as evaporation and        the recovery boiler, remain unchanged, so that the biorefinery        solution possibly required for further use of the lignin does        not require new process department.    -   Greater production with a smaller chemical cycle. When lignin is        recovered from the process, the amount of substance to be        combusted in the recovery boiler is decreased, because the        produced dry-matter amount per ton of pulp is lower, and the        chemical cycle is smaller. Then, a smaller boiler is needed, or        the existing boiler can burn a higher amount of black liquor dry        substance per ton of produced pulp.

Since all changes have an influence on many units, the following type ofmatters are to be taken into account in developing the process:

-   -   Overall steam production of the recovery boiler is decreased    -   A new separation department for lignin is needed    -   Chemicals of the oxygen stage or other delignifying stage may be        more expensive than the presently used    -   Effluent production changes and the conventional coupling can        not be used as such    -   There will be a need for sulfur-free or low-sulfur chemicals,        the price level of which is higher than that of        sulfur-containing chemicals.

The invention is especially applicable to pulp that is produced by asulfate method or its modification. The use of e.g. yield enhancingchemicals, such as anthraquinone, is possible in connection withcooking. The pulp is cooked in a continuous or a batch cook so that thedelignification is terminated at a high lignin-content level (kappanumber 50-120), but so that the defiberization point of the chips isreached. This kind of method is described e.g. in WO Patent ApplicationPCT/US10/57417. The present method comprises as a preferred embodiment aprocess coupling, in which the mechanical defibration of the completepulp stream after cooking can be omitted. Thus, the cooked pulp isdischarged from the digester into washing and then into the oxygen stagewithout mechanical treatment in a mechanical defibrating device, e.g. ablow line refiner. In a mechanical defibrator, mechanical work isapplied for releasing fibers from a chips matrix, which in the methodaccording to this embodiment is not needed. The fiber line naturallycomprises pumps, mixers and corresponding devices that are used for pulptransfer and for mixing chemicals and other substances into the pulp.Eliminating the mechanical defibrator is advantageous in view of energyeconomics, since the energy dissipation of the washer, pumps and mixersbetween the digester and the first oxygen stage is below 30 kW/adt,while a defibrator doubles that.

Screening of the pulp is more preferably accomplished after the oxygendelignification. According to an embodiment, the screen room is locateddownstream of the first oxygen stage, when there are several oxygenstages. According to an embodiment, the screen room is located betweenthe first oxygen stage and the subsequent wash. Alternatively, the pulpcan be screened also later, but preferably prior to washing followingthe second oxygen stage. The screening can also be divided into coarsescreening and fine screening, whereby the coarse screening takes placee.g. downstream of the first oxygen stage and the fine screeningdownstream of the second oxygen stage. Thus, for producing pulp to bebleached it is advantageous to merely wash the pulp after cooking priorto the first oxygen stage. Thus, the pulp discharged from the digesteris led, without mechanical defibration and without screening, viawashing into the first oxygen stage.

However, it is conceivable that knot removal from the pulp is performedafter discharging the pulp from the digester prior to the first wash.Thereby, in screening a knot reject separated from the main pulp streamis subjected to a gentle defibration. The defiberized reject stream canbe returned to the main pulp stream either to the knot separation feedor co-currently downstream of knot separation. It is essential that themain pulp stream is not defibrated mechanically, but only a side streamseparated therefrom (e.g. preferably less than 30%, most preferably lessthan 20%) is defibrated such that the knot fraction is in a suitablecondition for washing and subsequent treatment.

Washing refers to one or more washing stages and thus it takes place inone or more washing apparatuses.

According to an embodiment, the pulp discharged from the digester iswashed prior to the first oxygen stage, preferably using a single-drumwasher, such as a pressure drum washer produced by Andritz, i.e. a DrumDisplacer™ (DD)-washer, or a vacuum drum washer or a diffuser. A washer,in which the pulp gets pressed between drums and/or rolls in a so-callednip found e.g. in washing presses, is not suitable for pulp washingbetween the cook and the first oxygen stage. If knot removal is effectedprior to washing downstream of the digester, the use of a press ispossible, too. Washing downstream of the first oxygen stage can beaccomplished using conventional washing technique, which enables usingthe above mentioned washing apparatuses, and thus also a washing pressor a press, when screening has taken place prior to the oxygen stagewashing.

The first oxygen stage is a clear continuation to cooking, the purposeof which is to lower the kappa by 30-70 kappa units. Simultaneously alsothe reject content of the pulp decreases considerably, whereby in pulpscreening at some treatment step after the first oxygen stage noremarkable amounts of reject are removed, since removal would end theyield benefit. In the second oxygen stage the kappa number of the pulpis lowered to a level of 10-20, preferably to a value of 10-15 or less.

Studies have revealed that the reject content in the first oxygen stagedecreases even more than 95%, meaning that this coupling is especiallyefficient in lowering the reject content.

It is known that an oxygen molecule can only react with certain, i.e.phenolic lignin compositions. Under highly alkaline conditions (pH>10),an ionized lignin composition is decomposed and diminished as a resultof chain reaction, and dissolved. Oxygen reacts primarily with freephenolic hydroxyl groups of lignin forming carboxyl groups, whichconvert the lignin to water-soluble form. Simultaneously,depolymerization, i.e. degradation, of lignin takes place. Thesereactions lead to decrease in the number of phenolic hydroxyl groups oflignin and increase in the number of carboxyl groups, of lignin.

This phenolic lignin is removed in oxygen delignification in accordancewith oxygen delignification kinetics, but oxygen delignification doesnot have any notable effect on the amount of non-phenolic lignin. Forthis chemistry, the delignification level in the oxygen stage stops at alevel of 75-80%, even if oxygen treatments with the same chemicalconditions were coupled several in series and the treatment periodcontinued. Additionally, harshening of the conditions is clearly seen,above all as decreased quality of pulp and low yield. Thus, continuingdelignification after the oxygen stage requires chemicals that alsoreact with other lignin compositions and/or create new free phenolicgroups.

Experiments have also been made to find out whether the treatment ofpulp under acid conditions would activate the oxygen stage adequately,but it was noticed that an acid stage as such, without reaction withlignin, did not help in lowering the kappa. An acid stage intensifiesmass transfer, but does not change the chemical composition. Thus, ithas been discovered that after the oxygen stage the pulp is to besubjected to treatment with a bleaching chemical that reacts withnon-phenolic lignin. Suitable for this are so-called electrophilicbleaching chemicals. These typically comprise peracids, chlorine dioxideunder acid conditions, ozone or chlorine. Peracids comprise e.g.peracetic acid, Caron acid, persuiphuric acid, and peroxomolybdates.Treatment of pulp with these chemicals can be performed under conditionsthat are known per se and presented in handbooks for the field. Chlorinedioxide treatment is typically performed at a temperature of 40-90° C.,at a pH of 1.5-5.5 and during 1-10 minutes, but longer periods are alsopossible. A typical chlorine dioxide dose in the treatment is 2-20kg/adt. Typical conditions for peracid treatments are: temperature50-90° C. and time 30-120 minutes. The amount of chemical added into thepulp depends on the peracid used, but typically it is 2-20 kg/adt.

Treatment with ozone is typically effected pressurized under a pressureof 5-15 bar, preferably at a consistency of 5-35%, more preferably at aconsistency of 7-18%, so that ozone-containing gas having anozone-content of 8-18 weight percent, preferably 10-15%, is introducedinto the pulp at one or several points, preferably via one or moremixers. Said gas is a mixture of ozone and oxygen that acts as carriergas. The amount of ozone dosed into the pulp is 2-8 kg/admt. The pH isin the acid range, typically 1-5. Ozone bleaching is performed eitheronly in mixers allowing a long retention or so that the mixture of pulpand ozone with carrier gas is introduced into a reaction vessel, whereinthe desired reaction retention is arranged.

After electrophilic chemical treatment, the pulp is delignified a secondtime with oxygen, the purpose of which is lowering the kappa number to atargeted kappa number of 10-20 or in some cases even lower than 10.After this the pulp can be bleached in a conventional total chlorinefree (TCF) bleaching process or in an elementary chlorine free (ECF)bleaching process.

The present invention is described in more detail by means of anembodiment according to the invention and with reference to theaccompanying schematic drawings, in which:

Fig. is a schematic illustration of a preferred oxygen delignificationof high kappa pulp without lignin separation; and

FIG. 2 is a schematic illustration of a preferred exemplary embodimentof the method according to the present invention.

The digester equipment comprises one or more pulp digesters. Wood pulpcooked to a kappa number of 50-120, typically 60-100, is taken from thebottom of the digester 1 via blow line 2 into a blow tank 3, from wherethe pulp is pumped via line 4 into a washer 5. The washer is typically asingle drum washer, such as e.g., a pressure drum washer produced byAndritz, i.e. a Drum Displacer™ (DD)-washer, or a diffuser or a suctiondrum washer. The washed pulp is led into an oxygen stage. The pulpdischarged from the digester is not defibrated mechanically andtherefore the blow line is devoid of a mechanical defibrator downstreamof the digester. Also, the pulp is not screened here prior to the firstoxygen stage. The blow tank 3 is not inevitable, but the pulp can alsobe taken directly into the washer 5, depending on the type of washer andequipment coupling, as known per se.

Black liquor is discharged from the digester via line 33.

In this example, the first oxygen stage comprises two reactors 7 and 8,i.e. it is a two-step stage, but it can also be a one-step or multi-stepstage, as described above. In this oxygen stage the kappa number of thepulp typically decreases by 30-70 units.

From the first oxygen stage the pulp is led via line 9 to a screeningroom 10. The pulp is led further via line 11 to washing 12 downstream ofthe oxygen stage, where the washer can instead of a single drum washeror a diffuser be also a press.

Next the pulp is treated with an electrophilic chemical in a reactionvessel 14, wherein the washed pulp is discharged from the washingapparatus 12 via line 13. The pulp is treated after the first oxygenstage with such a bleaching chemical that reacts with non-phenoliclignin. Suitable for this are so-called electrophilic bleachingchemicals. These typically comprise peracids, chlorine dioxide underacid conditions, ozone or chlorine. This treatment adequately promotesthe delignifiction of pulp cooked to a high kappa for the actualbleaching process.

After treatment in the reaction vessel 14 the pulp is discharged vialine 15 into a washer 16, which can be a similar apparatus as the washer12. After this the washed pulp is taken via line 17 to a second oxygenstage, which similar to the first oxygen stage comprises two steps 18and 19. In this oxygen stage the kappa number of the pulp is lowered toa level of 10-20, preferably 10-15 or less.

The pulp from the second oxygen stage is led via line 20 into a washer21, which can be single drum washer or a diffuser or also a press. Thewashed pulp is taken via line 27 into a bleaching process. The pulp thathas been cooked to a high kappa has after the cook been treated so thatits kappa number has been decreased to a level advantageous forbleaching, but simultaneously the quality of the pulp has remained at agood level, since no mechanical defibration has been needed.

FIG. 1 illustrates that the pulp is washed as known per se in accordancewith counter-current principle. Thereby the filtrate from the latteroxygen stage washing 21 is led counter-currently via line 22 into awasher 16 as washing liquid. Because treatment 14 with an electrophilicchemical is carried out under acid conditions, a portion of the filtratefrom the downstream washer is discharged from the process via line 24,but a portion is taken via line 23 to the first oxygen stage washer 12as washing liquid. The filtrate discharged from the washer 12 is led vialine 25 to a washer 5 downstream of the digester, the filtrate fromwhich washer is led via line 26 to digester washing into digester 1.Additionally, water or other washing liquid is introduced into thewashers 21 and 12 via lines 32.

In connection with FIG. 2, the same reference numerals as in FIG. 1 havebeen used where applicable. Wood pulp cooked to a kappa number of50-120, typically 60-100, is taken from the bottom of the digester 1 viablow line 2 into a blow tank 3, from where the pulp is pumped via line 4into one or several washers 5. The washer is typically a single drumwasher, such as e.g., a pressure drum washer produced by Andritz, i.e. aDrum Displacer™ (DD)-washer, or a diffuser or a suction drum washer.Before that, the cooked pulp is washed at the bottom of the digester bya so-called digester wash. The E₁₀-value of the washing equipment forcooked pulp, i.e. digester wash and the washer 5, is at least 8,preferably at least 10.

The washed pulp is led into an oxygen stage. In this embodiment, thefirst and the second oxygen stage have one reaction vessel 8 and 19,respectively, but there may be more vessels, as in FIG. 1. A furtherdifference compared to the arrangement of FIG. 1 is the treatment of thefiltrate of the washer downstream of the oxygen stage. In the oxygenstage, especially in the first treatment, a large amount of lignin isreleased. When the amount of removed lignin is e.g. 40 kappa units, theamount of organic material being released from the pulp becomes high.With said kappa reduction, the proportion of e.g. lignin only isapproximately 7% of the dry matter of the pulp, and thereto carbohydratelosses. Due to this, the COD-amount of the solution, which characterizesthe organic substance in the filtrates, may rise to 100-200 kg/adt as aresult of lignin. In FIG. 2, the filtrate of the washer 12 downstream ofthe first oxygen stage 8 contains abundantly lignin, especially if thekappa number in the reaction vessel 8 has been decreased by 30-70 units.Therefore, lignin is separated from the filtrate 25 of the washer 12 instage 29 by a method that may be known as such, e.g. by precipitating orfiltering. Various lignin separation methods can be applied here. It isadvantageous that the first oxygen treatment uses sodium hydroxide asalkali instead of oxidized white liquor for avoiding sulfur compounds.The lignin fraction is recovered in low-sulfur form, and thus it isadvantageously used as raw material for chemical industry. As a resultof lignin separation, the filtrate fluid is cleaned, and the filtrate isused at the washer 5 downstream of the digester and upstream of theoxygen treatment, from which washer the filtrate is taken via line 32into digester washing. The lignin-containing filtrate 25 is in FIG. 1used as washing liquid for pulp at the washer 5 without separatinglignin.

In the following, the results of a series of experiments are shown inform of a table, in which the pulp was treated in connection with theoxygen stage with various sequences. The experiments were conducted forScandinavian soft wood pulp (SW). From the data in the table it isnoticed that pulp treated with either acid peroxide (Caron acid) orchlorine dioxide has been delignified to a kappa number level ofapproximately 15. The kappa number of pulps treated with hydrochloriteor acid only or not treated at all, was, in turn, at a level of 20. Theresults confirm the fact that the kappa reduction of high kappa pulp inthe oxygen stage is at the most approximately 80%, For re-activating thepulp for the oxygen stage, a treatment with an electrophilic chemical(Caron acid or chlorine dioxide) is required. This kind of pulp can bebleached to high brightness.

Initial pulp Kappa number — 79.1 Brightness % ISO 20.4 Reject content %(on pulp) 28.7 Sequence O—A—O O—Px(H₂SO₅)—O O-D₀-O O—H—O O—O_(p) 1stOxygen Reactor type — MC-mixer MC-mixer MC-mixer MC-mixer MC-mixerdelignification stage Consistency % 10 10 10 10 10 Temperature ° C. 100100 100 100 100 Retention time min 10/50 10/50 10/50 10/50 10/50O₂-pressure bar(g) 6.9/5.5 6.9/5.5 6.9/5.5 6.9/5.5 6.9/5.5 NaOH-charge %(on pulp) 6.5 6.5 6.5 6.5 6.5 MgSO₄-charge % (on pulp) 0.15 0.15 0.150.15 0.15 Final pH — 11.8 11.1 11.4 11.4 11.2 Kappa number — 32.8 31.731.9 29.6 30.7 Viscosity mL/g 1020 1030 1031 1011 1003 Brightness % ISO28.6 29.5 30.3 31 29.1 Yield % (on pulp) 89.59 89.91 88.8 89.9 88.1Reject content % (on pulp) 0.08 0.12 0.08 0.04 0.08 COD kg/BDMTp na164.6 175.4 183.3 191.2 Intermediate Type A Px D₀ H — activation stageConsistency % 10 10 10 10 — Temperature ° C. 60 70 70 70 — Retentiontime min 60 60 15 30 — Chemical charge % (on pulp) H₂SO₄ 0.95 H₂SO₅ 2.0ClO₂ 0.38 NaOCl 0.5 — Chemical charge % (on pulp) — NaOH 4.9 H₂SO₄ 0.88NaOH 0.69 — Final pH — 3.5 2.9 2.9 11.2 — Kappa number — na 26.7 28.528.3 — Viscosity mL/g na 877 na 1018 — Brightness % ISO na 37.8 na 36.4— 2nd Oxygen Reactor type Autoclave Autoclave Autoclave AutoclaveAutoclave delignification stage Consistency % 10 10 10 10 10 Temperature° C. 100 100 100 100 100 Retention time min 85 85 85 85 85 O₂-pressurebar(g) 5.5 5.5 5.5 5.5 5.5 NaOH-charge % (on pulp) 3.5 3.5 3.5 3.5 3.5MgSO₄-charge % (on pulp) 0.15 0.15 0.15 0.15 0.15 H₂O₂-charge % (onpulp) — — — — 0.6 Final pH — 11.5 11.5 11.6 12 11.9 Kappa number — 20.814.8 16.2 18.8 18.9 Viscosity mL/g 900 791 915 892 856 Brightness % ISO37.2 48.2 47.1 37.2 40.9 Yield % (on pulp) 96.6 96.5 95.9 96.2 97.5 CODkg/BDMTp na 76.5 59.8 54.3 67.6

The present method can provide an efficient washing and delignifyingmethod for producing pulp from the high kappa pulp so that the qualityof the pulp is not deteriorated and that the yield is good.

Although the above description relates to an embodiment of the inventionthat is in the light of present knowledge considered the mostpreferable, it is obvious to a person skilled in the art that theinvention can be modified in many different ways within the broadestpossible scope defined by the appended claims alone.

1. A method of treating chemical pulp comprising: alkaline cooking cellulosic fibrous material to produce a pulp having a kappa number in a range of 50 to 120; washing the pulp; feeding the washed pulp to an oxygen stage in which the wash pulp is treated in the presence of oxygen and alkali to remove lignin; washing the oxygen-treated pulp such that the kappa number by at least at least 30, and separating lignin from wash filtrate generated by the washing of the oxygen-treated pulp, and applying the separated wash filtrate as a washing liquid used in the washing of the pulp.
 2. The method according to claim 1, wherein the washing of the pulp is with a washer having an E₁₀-value of at least
 8. 3. The method according to claim 1, the decrease of the kappa number is at least
 35. 4. The method according to claim 1, wherein the oxygen stage includes oxidizing with sodium hydroxide.
 5. The method according to claim 1, further comprising first screening the pulp downstream of the oxygen stage.
 6. The method according to claim 5, wherein the oxygen stage is a first oxygen stage and the method further comprises screening the washed pulp downstream of the first oxygen stage and upstream of a second oxygen stage.
 7. The method according to claim 1, further comprising knot-separation of the pulp prior to treating the pulp with the oxygen stage.
 8. The method according to claim 1, wherein the lignin is separated by precipitating the wash filtrate.
 9. The method according to claim 1, wherein the alkaline cooking includes a sulfate process.
 10. The method according to claim 1, wherein the alkaline cooking includes a soda process.
 11. The method according to claim 1, wherein the alkaline cooking is in a digester vessel and further comprising discharging the pulp from the digester vessel and feeding the to washing without treatment to a mechanical defibrator between the digester and the washing,
 12. The method according to claim 1, further comprising recovery the separated lignin.
 13. A system for treating pulp produced from wood chips in a cooking apparatus wherein the wood chips are cooked chemically to a kappa number in a range of 50 to 120, said system comprising: a continuous digester vessel including an upper inlet configured to receive cellulosic fibrous material and a lower outlet configured to discharge pulp produced from the cellulosic fibrous material in a cooking process performed in the digester vessel; a first pulp washer coupled to the lower outlet of the digester vessel to receive the pulp, wherein the pulp washer includes an outlet for washed pulp; an oxygen stage coupled to the outlet of the pulp washer to receive the washed pulp, the oxygen stage configured to treat the washed pulp in the presence of oxygen and alkali and including an outlet for the treated pulp; a second pulp washer coupled to the oxygen stage and configured to wash the treated pulp from the oxygen stage; a first filtrate conduit for wash filtrate from the second pulp washer; a separator coupled to the first filtrate conduit, wherein the separator includes a lignin outlet for lignin separated from the wash filtrate and a treated wash filtrate outlet, and a wash liquid conduit coupled to the treated wash filtrate outlet and to a wash liquid inlet to first pulp washer.
 14. The system according to claim 13, wherein: the first washer includes a pulp inlet connected to a conduit which is connected to the lower outlet of the continuous digester vessel, and the outlet for the washed pulp is connected to a conduit which is connected to a pulp inlet the oxygen stage; the second pulp washer includes a pulp inlet connected to a conduit which is connected to the pulp outlet of the oxygen stage, and the separator is arranged in the wash liquid conduit.
 15. The system according to claim 13, wherein the first pulp washer has an E₁₀-value of at least
 8. 16. A method to pulp cellulosic fibrous material comprising: alkaline cooking cellulosic fibrous material in a continuous digester vessel to produce a pulp having a kappa number in a range of 50 to 120; washing the pulp discharged from the digester vessel; treating the washed pulp in the presence of oxygen and alkali; washing the oxygen-treated pulp such that the kappa number is recued by at least at least 30; separating lignin from wash filtrate generated by the washing of the oxygen-treated pulp, and using the separated wash filtrate as a washing liquid in the washing of the pulp from the digester vessel.
 17. The method according to claim 16 further comprising screening the pulp downstream of the oxygen stage.
 18. The method according to claim 16 wherein the treatment of the washed pulp includes treating the washed pulp in a first oxygen stage in the presence of oxygen and an alkali, screening the washed pulp downstream of the first oxygen stage to extract wash liquor from the washed pulp, and treating the screen washed pulp in a second oxygen stage in the presence of oxygen and an alkali. 